1. Field of the Invention
The present invention relates to a method and a device for driving a display device of an active matrix system, especially of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device (hereinafter, referred to as the "LCD device") of an active matrix system includes a display panel having an active matrix substrate on which pixel electrodes each as a display unit are formed in a matrix. The display panel further includes a counter substrate opposed to the active matrix substrate and a display medium such as a liquid crystal provided therebetween. The counter electrode includes a common electrode opposed to the pixel electrodes. The active matrix substrate includes a plurality of scanning electrodes (each provided as a scanning line or a gate line) located parallel to each other and a plurality of signal electrodes (each provided as a signal line or a source line) located parallel to each other and perpendicular to the scanning electrodes. Each of the pixel electrodes is formed in an area enclosed by two adjacent scanning electrodes and two adjacent signal electrodes, and connected to the corresponding signal electrode via a switching element (for example, a thin film transistor; hereinafter, referred to as the "TFT") and connected to the corresponding scanning electrode via a gate. The common electrode usually includes a single conductive layer opposed to all the pixel electrodes. A pixel includes a pixel electrode, an area of the common electrode opposed to the pixel electrode, a switching element, and an area of the display medium corresponding to the pixel electrode. The pixel may include a storage capacitor to improve display quality.
In such an LCD device, the state of the display medium provided between the active matrix substrate and the counter substrate is optically changed by applying a voltage between at least one of the pixel electrodes and an area of the common electrode opposed to the pixel electrode, and thus display is performed.
Methods for driving the common electrode are generally classified into two. One is a method using a DC voltage, and the other is a method using an AC voltage having a reference voltage as a central voltage.
Briefly referring to FIG. 6, the driving method of the common electrode using a DC voltage will be described. FIG. 6 illustrates the relationship among waveforms of a voltage V.sub.COM for driving the common electrode, an ON voltage V.sub.GH of the scanning electrode, an OFF voltage V.sub.GL of the scanning electrode, and a voltage V.sub.0 for driving the signal electrode used to apply a maximum voltage to the liquid crystal. Driving which is performed using a DC voltage both for the ON voltage and the OFF voltage of the scanning electrode is referred to as the "DC voltage driving of the scanning electrode". In FIG. 6, V.sub.0.sup.+ indicates a voltage applied to the liquid crystal when the pixel electrode is charged positively with respect to the common electrode (namely, the potential difference between the voltages V.sub.0 and V.sub.COM). V.sub.0.sup.- indicates a voltage applied to the liquid crystal when the pixel electrode is charged negatively with respect to the common electrode (namely, the potential difference between the voltages V.sub.COM and V.sub.0). The time when the pixel electrode is charged positively with respect to the common electrode is referred to as the "positive time", and the time when the pixel electrode is charged negatively with respect to the common electrode is referred to as the "negative time". The absolute values of the voltages V.sub.0.sup.+ and V.sub.0.sup.- are approximately equal to each other. In such a case, a signal line driver (data driver) is required to have an output dynamic range of .vertline.V.sub.0.sup.+ .vertline.+.vertline.V.sub.0.sup.- .vertline..
Briefly referring to FIG. 7, the driving method of the common electrode using an AC voltage will be described. FIG. 7 illustrates the relationship among waveforms of the voltage V.sub.COM for driving the common electrode, the ON voltage V.sub.GH of the scanning electrode, the OFF voltage V.sub.GL of the scanning electrode, and the voltage V.sub.0 for driving the signal electrode used to apply a maximum voltage to the liquid crystal. FIG. 8 illustrates the voltages V.sub.GH, V.sub.GL and V.sub.0 with respect to the voltage V.sub.COM (namely, the potential difference between each of the first three voltages and the voltage V.sub.COM). As is apparent from FIG. 8, in the driving method using an AC voltage, an output dynamic range of the signal line driver can be expanded by the difference between the high level and the low level of the voltage V.sub.COM. In other words, the output dynamic range of the signal line driver can be narrowed by the above-mentioned difference. This is advantageous in, for example, enhancing the driving speed of the signal line driver, reducing production cost, and reducing power consumption when the LCD device is used as a module. Accordingly, the driving method using an AC voltage is widely used today.
In a conventional driving method of the common electrode using an AC voltage, the ON voltage V.sub.GH and the OFF voltage V.sub.GL of the scanning electrode are both a DC voltage as is illustrated in FIG. 7. Accordingly, with respect to the common electrode, the ON voltage V.sub.GH and the OFF voltage V.sub.GL of the scanning electrode are both an AC voltage as is illustrated in FIG. 8. This is not a serious problem in the case of, for example, a conventional LCD device using TFTs in which one of two electrodes of the storage capacitor is formed on the common electrode, since the degree of deterioration of the display quality is low. In such a storage capacitor, the opening ratio (the ratio of an area for transmission therethrough of light with respect to a display area) is lowered by the storage capacitor. However, the reduction in such a ratio is not a serious problem in the conventional LCD device without very high display protection.
As the display device has been enlarged and the display precision has been enhanced in recent years, an absolute area of each pixel has been reduced and an area which does not transmit light such as a switching element such as a TFT, a scanning electrode and a signal electrode has been enlarged. As a result, the whole image plane is more and more darkened, which proposes a serious problem. In order to solve this problem, a structure in which the storage capacitor is formed on the scanning electrode (Cs-ON gate structure) is adopted to avoid adverse influence of the storage capacitor on the luminance. In this structure, one of the electrodes of the storage capacitor is, for example, connected to the scanning electrode or connected to an independent electrode formed at an end of a display panel using an independent wire provided for the storage capacitor. If the scanning electrode is driven by a DC voltage in such structure, the display quality is drastically deteriorated to possibly propose a serious problem in the practical use of the LCD device.
In order to restrict such deterioration of the display quality to a minimum extent, a floating gate driving method was developed. Hereinafter, the floating gate driving method will be briefly described with an example of a case of 3 bits and 8 gray levels (data 0 through data 7) with reference to FIGS. 9 through 11.
FIG. 9 illustrates an example of waveforms of driving voltages obtained by the floating gate driving method. V.sub.COM indicates a voltage for driving the common electrode. V.sub.0 and V.sub.7 indicate the voltage of a gray-level power supply of a signal electrode driving circuit for data 0 and data 7, respectively. V.sub.GH and V.sub.GL indicates an ON voltage supplied by an ON voltage supply circuit and an OFF voltage supplied by an OFF voltage supply circuit of a scanning electrode driving circuit, respectively. As is apparent from the waveforms of the voltages V.sub.0, V.sub.7, V.sub.GH, V.sub.GL and V.sub.COM, the signal electrode, the scanning electrode and the common electrode are all driving by an AC voltage using a certain voltage as a central voltage in this method. Voltages V.sub.1 through V.sub.6 of other gray-level power supplies of the signal electrode driving circuit respectively corresponding to data 1 through data 6 depend on a voltage between the voltages V.sub.0 and V.sub.7. The voltages V.sub.GH and V.sub.GL are set to have a waveform having an equal amplitude.
FIG. 10 illustrates the voltages V.sub.0, V.sub.7, V.sub.GH, and V.sub.GL relative to the voltage V.sub.COM (namely, the potential difference between each of the first four voltages and the voltage V.sub.COM). As is apparent from FIG. 10, the display panel obtains the same effects in the case when the common electrode, the gray-level power supplies, and the scanning electrode are driven by the floating gate driving method and in the case when these electrodes are driven by the conventional method in which the common electrode is driven by a DC voltage and the gray-level power supplies and the scanning electrode are driven so that the voltages V.sub.0, V.sub.7, V.sub.GH and V.sub.GL have the same relationship as the relationship shown in FIG. 10.
FIG. 11 is a circuit diagram of a conventional floating gate driving circuit, which is known. In the floating gate driving method, the scanning electrode driving circuit and a power supply for the circuit (scanning electrode means) is entirely separated from the other electric systems when a DC current flows, and the power supply for the scanning electrode driving circuit rides on the waveform on the driving voltage of the common electrode. As is illustrated in FIG. 11, a control signal G.sub.SF (scanning start pulse) and a control signal G.sub.CK (scanning clock pulse) which are to be supplied to the scanning electrode driving circuit are separated from a control signal generating circuit and supplied to the scanning electrode driving circuit as a control signal G.sub.SP ' and a control signal G.sub.CK '. An output from a common electrode driving circuit 101 including an operational amplifier (not shown) is supplied to the scanning electrode driving circuit as an ON voltage V.sub.GH, an OFF voltage V.sub.GL and driving supply voltages for elements of a logic system via a transformer 102. A voltage V.sub.s is, for example, a supply voltage of +5 V or more, and the control signals G.sub.SP and G.sub.CK have a level between, for example, +5 V and the ground.
Conventionally, photocouplers 103 or the like are required to separate the control signals G.sub.SP and G.sub.CK from the other electric systems. Provision of the photocouplers 103 makes the structure of the floating gate driving circuit complicated and causes a rise in production cost. Further, the theory concerning the floating gate driving method is qualitative, as is apparent from FIGS. 9 and 10. For these reasons, the floating gate driving method has not been actively developed.
Moreover, as the display panels have been improved in display precision and enlarged in size, problems of the circuit such as shown in FIG. 11 have become serious. Practically, the load characteristics of the display panel has been become larger than before. Accordingly, a waveform of the voltage for driving the common electrode, a waveform of the driving voltage actually applied to the common electrode in the display panel, and a waveform of the voltage for driving the scanning electrode driving circuit have become more and more distorted. As a result, the display quality is deteriorated. Further, the voltage of the pixel electrode of the display panel is influenced by the distortion of the waveform of the voltage for driving the common electrode. This undesirably causes slight fluctuations in the gray level.